Iodotyrosine deiodinase. The steapsin-solubilized enzyme will be further purified by preparative gel electrophoresis and affinity chromatography and will be characterized in regard to unit molecular weights, sulfhydryl content, flavin-binding and spectral properties in presence of substrate reducing agents and sulfhydryl blocking agents. Isolation, purification and characterization will be attempted of the putative electron carrier intermediate responsible for NADPH-mediated reduction of the enzyme, with particular emphasis on the stabilization of this factor during purification. Conversion of L-T4 to T3. Subcellular location of the enzyme(s) catalyzing T4 to T3 conversion will be investigated with special attention to the plasma membrane fraction; different preparative methods will be tried as Neville's procedure in our hands has resulted in substantial losses of activity. Attempts will be made to solubilize and purify the enzyme from the 2000g pellet of kidney homogenate. The possible role of thyroid NADPH-cytochrome C reductase as an enzymatic step common to organic binding of iodine and also to deiodination of iodotyronine will be explored by studying the effects of purified specific antibody directed against this enzyme on microsomal hydrogen peroxide formation, organic binding of iodine, and deiodination of iodotyrosines as well as possible effects of antibody upon deiodinase activity of cholate-solubilized preparation. BIBLIOGRAPHIC REFERENCES: Goswami, A. and I.N. Rosenberg. NADPH-responsiveness of soluble thyroid iodotyrosine deiodinase, in Thyroid Research, J. Robbins & L.E. Braverman, editors, American Elsevier Publishing Co., N.Y., 1976, p. 162. Chiraseveenuprapund, P., U. Buergi, A. Goswami and I.N. Rosenberg. Formation of triiodothyronine from L-thyroxine in rat kidney homogenate, in Thyroid Research, J. Robbins and L.E. Braverman, editors, American Elsevier Publishing Co., N.Y., 1976, p. 244.